Stability evaluation of potentially toxic elements in MSWI fly ash during carbonation in view of two leaching scenarios

Carbonation treatment (CT) by alkaline fly ash (FA) affects the stability of potentially toxic elements (PTEs). This study investigated the leachability and environmental risk of six PTEs contained in FA during natural and accelerated carbonation (NC, AC) using two typical leaching scenarios with distilled water (DW) and acetic acid (M). The leaching of Pb/Cu/Cr/Ni in solidified/stabilized FA decreased due to CT in DW leaching, but the leaching of Pb/Zn/Cu/Cd increased due to CT in M leaching. The leaching of the six PTEs (especially Pb/Cd) in AA leaching was significantly higher than that in DW leaching. CT was a promoting factor to increase the environmental risk level of PTEs in FA leachate, especially in AA leaching with H+ input. In the early stage of NC, under DW leaching tests, the environmental risk level of PTEs in FA leachate can be weakened due to the formation of carbonate minerals in the FA matrix. However, excessive NC increases the environmental risk of leached PTEs due to the decalcification of carbonate minerals. Both NC and AC increased the potential environmental risk of PTEs contained in the carbonated FA matrix. The nucleation and dissolution of carbonate minerals were interdependent with the immobilization and leaching of PTEs, which played a dominant role in the Cr and leaching tests respectively. They jointly affected the occurrence behavior of PTEs in the FA matrix in CT tests and the leachability of PTEs in leaching tests. This study demonstrates that it is more scientific to evaluate the leachability of PTEs in carbonated FA according to the actual disposal scenarios. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Carbonation treatment by alkaline fly ash affects the stability of potentially toxic elements (PTEs), increasing the leachability of PTEs, especially in acetic acid leaching. The formation of carbonate minerals in the ash matrix can decrease the environmental risk level of PTEs, but excessive carbonation can lead to an increase in environmental risk due to decalcification. Both natural and accelerated carbonation increase the potential environmental risk of PTEs in the carbonated fly ash matrix.